What does the notation sup dist mean in distributional RL?
It doesn't seem that it is a "proper" symbol. 

I guess that $\sup$ simply refers to the supremum, that is, you want to select actions that maximize the quantity that comes to the right of $\sup$, while $\text{dist}$ is simply a proxy for any possible distance between distributions. For example, you can replace $\text{dist}$ with the Kullback-Leibler divergence or with the mutual information.